Concrete Structures
For years, concrete has been one of the most basic building materials used in the construction world. One of its most common uses is in a support role for highways, bridges, and buildings. In this role, it is usually found in the form of a column, with both a base that anchors it to the ground and a top that incorporates the deck of the structure that it supports, or in the form of a beam that is used to support a load and spans between columns or other supporting systems.
While concrete alone has fairly good compressive strength and structural characteristics, it became apparent to engineers and designers that a method of reinforcing the concrete was critical as the columns began to be designed for larger and larger loads. The chief means of reinforcing the concrete came from the other most common material in the construction world - steel. In various forms, steel was incorporated in the columns (internal reinforcement) to increase their tensile and bending load carrying capacity. If properly employed, the steel could greatly increase the strength and ductility of the column. The internal steel reinforcements appeared to be the answer. As time passed, however, it became apparent that there were many problems with the steel reinforced concrete columns.
First, the success of the reinforcing steel depends greatly on the proper execution of its installation. One of the main types of steel reinforcements is hoop steel, which is pieces of rebar or steel strap that are bent into hoops and welded or tied to the vertical rebar reinforcing members. When properly welded together and to the vertical members, the hoop steel substantially improves the column's ability to withstand dilation, tremors, and shocks associated with seismic disturbances. If the hoop steel is not properly welded, or attached to the vertical members, the transverse tensile loads from the seismic disturbances will cause the column to spall, which will lead to large chunks of concrete being dislodged from the column as the hoop steel is forced open. The failure of several major concrete columns in a concrete column supported interstate (I-880) in California during a recent earthquake showed that much of the hoop steel reinforcing members in the columns were not welded during installation. Contractor documentation revealed that these poor installation practices were a common occurrence in California and other states (pre-1975), thus thousands of in-use concrete column supported structures are deficient in their load carrying capacities and seismic performance.
It has also been shown that under typical column or beam stress states, the poor tensile strength of concrete initiates failures at the surface of the column or beam.
A second major problem involves the inherent nature of steel and concrete, they are both readily susceptible to corrosive elements such as water and their environment (acid rain, road salts, chemicals, oxygen, etc.). Concrete shows the effects of environmental attack by pitting, and spalling, which leads to severe cracking and a marked reduction in strength. Steel not only succumbs to chemical attack (rust), but during the process undergoes a physical transformation in size (increases). Rusting reinforcing steel in concrete columns expands to the point that the internally created stresses are so large that they crack the concrete to such an extent that often large pieces of concrete are displaced from the column. The net result is a dramatically weaker structure.
Steel Construction
Steel is not only used by the construction world as a reinforcing agent but also as a primary building agent. But this fact does not change the way steel reacts to the environment. Steel is very susceptible to environmental attack and great measures must be taken, in the form of paints and surface treatments and alloys, in order to prolong the life of the steel.
There are thousands of in use steel structures that are poorly maintained and in need of rehabilitation. Many of these structures have deteriorated to the point that welding on new steel to reinforce the structure would add so much weight that the structure would collapse. Wood Structures
Much like concrete and steel, wood structures also fall prey to the environment. This takes place in the form of rot. As wood rots, its structural integrity is reduced resulting in a dramatically weaker structure. While wood is not commonly used in large structures such as (new) bridges and highway overpasses, it remains a primary building material, especially in and around marine environments and in small rural bridges. Similar to steel, there are many wooden structures that are poorly maintained and in need of rehabilitation. In addition to bridge structures, telephone poles represent a very large use of a wood structure as a load bearing element. Every year, thousands of poles need to be replaced due to rot, especially near or below the ground. Instead of replacement, these structures could be repaired using the appropriate jacking technique.
In and around tidal zones, environmental attack is much more apparent. In particular, concrete, wood, and steel support columns, beams, and structures that are in a marine environment (such as docks, offshore platforms, etc.) exhibit dramatically shorter life times as they fall prey to corrosion, tidal erosion, and marine bore attack. Support columns in a relatively close proximity to these marine areas also exhibit a reduced life span as the effects of the corrosive environment spread.
In an era of expanding population, increased highway travel, constant earthquake threats, increased shipping vehicle loads, and continuing environmental decay, now more than ever, there exists a need to rehabilitate these structures in a fast, inexpensive, safe, and environmentally clean way that will last well into the future. The key to the successful rehabilitation of these structures will be to minimize the disruption of the activities that occur over and around the structures. Simply, this means not shutting down traffic lanes as bridge support columns are retrofitted, piers as pilings are retrofitted, etc. The ability to fix in place will be instrumental in the success of these programs.
As the idea of an external reinforcement for support columns gained acceptance, the first attempts used steel jackets as a reinforcing means. These jackets consisted of large pieces of steel plate that were rolled to the diameter of the column in question. A crane was then used to position the pieces around the column and the pieces were butt welded together. This solution had many problems, the most important being the weight of the pieces. The plate had to be fairly thick so that a good weld could be achieved and so that the pieces would not bend and kink as they were being lifted from the truck on which they were transported. The welded butt joint gave no tolerance to the column, thus requiring the additional step of grouting between the jacket and column to accommodate typical field tolerances. This heavy weight necessitated the use of heavy equipment to both transport and install the pieces. The large equipment led to many problems as multiple traffic lanes on the interstates had to be closed in order to install the plates. The weight of the plates also posed a safety issue for the workers.
Based on the critical jacket thickness for welding and the characteristic material properties of steel, these jackets were actually too stiff for their intended purpose. The now stiffened column structure would actually attract additional load during a seismic disturbance and change the designed fundamental natural frequency of the structure, thus creating new structural problems and increasing the likelihood of failure. Another problem came again from the nature of steel, as it corrodes very easily. Although steel itself is inexpensive, the above mentioned structural, application, and maintenance problems all contributed to a high system cost.
As the knowledge base and use of composite materials increased, it became apparent that composite materials offered a potential solution to the decaying or poorly constructed concrete column problem and the problems associated with the steel reinforcing jackets. These materials could offer dramatic increases in strength and are impervious to the environmental attack that destroys the steel and concrete. Additionally, the tailorability of the composite allows for the application of strength in specific (fiber) directions with or without the introduction of stiffness, depending on the desired affect.
U.S. Pat. No. 4,786,341 describes a process of wrapping a concrete column with a resin impregnated fiber. Essentially, this is filament winding around an existing structure in the field. While the final composite encasing is of adequate strength, the process is excessively time consuming, prohibitively costly, produces a composite with a high percentage of voids (3%-5%), and exposes large amounts of chemical byproducts of the resin to the workers and the environment. Additionally, applying the reinforcement near the column ends is very difficult. In this case, field conditions will heavily influence the composite quality and its associated material properties.
U.S. Pat. No. 5,043,033 describes a process of wrapping a concrete column with a fiber tape, encasing the outside with a resinous substance to create a shell, and injecting the gap between the concrete and the fibers with a hardenable liquid. While the final composite encasing is of adequate strength, the composite is susceptible to air entrapment, and the process is excessively time consuming, and prohibitively costly, especially including the fluid injection (pressure grouting) step. Again, field conditions will greatly influence the final material properties.
U.S. Pat. No. 5,218,810 describes a process where a fibrous preform of considerable width is pre-impregnated with a resin and wrapped around the concrete column to form a composite reinforcement. While this process theoretically showed an improvement in time versus the two previously cited patents, it still did not solve many problems. Although the actual wrapping time was theoretically reduced, the necessary equipment set-up and removal times were still very long as was the time to adequately impregnate the fibers with resin, thus rendering the process prohibitively costly. Field tests showed that handling the `wet-preg` was very difficult, especially under windy and dirty conditions. Additionally, the composite was of an inferior quality (5%-10% voids typical in this type of lay-up process) and there was still an unreasonable exposure of the environment and the workers to chemical byproducts of the impregnating resin process.
The previously mentioned methods all suffer from multiple problems. In all cases, the excessive time requirements for equipment set-up, removal, and the actual wrapping time for the process led to costs that were excessive. The final quality of the composite members is also brought into question. Each method is extremely susceptible to air entrapment, incomplete fiber wetting, and contamination during the handling and subsequent lay-up of the impregnated fibrous preform. The air and debris entrapment experienced during field installation causes voids that substantially weaken the reinforcing capabilities of the composite material. Constantly varying field temperatures influence the fundamental chemistry of the impregnating resin, again leading to wide variations in the final retrofit system quality. Finally, making the composite on the target structure leaves no room for error. If problems occur during installation, the costly process of removing the composite from the column must be undertaken and the entire process must be repeated.
In the case of `wet-preg` in wet lamination, compaction forces must be applied via a vacuum bag before any of the reinforcing layers begin to cure or gel. Time constraints of the wet process, gravity effects of a "total thickness, ungelled system", and typical bag leaks on cracked concrete make `wet bagging` in the field a completely unmanageable task.
It is the objective of this patent to provide an improved process for the reinforcement of concrete, wood, and steel columns, beams, and structures preferably with composite materials that is fast, inexpensive, predictable, repeatable, environmentally sound, and accommodating to typical field tolerances.
It is an additional objective of this patent to provide a reinforcement apparatus of composite materials of superior quality, versus other composite articles.
It is a further additional objective of this patent to provide an improved means of manufacturing said composite materials.
These and other objectives of the invention will be apparent to those skilled in this art from the detailed description of a preferred embodiment of the invention.